Plasma-based seeded soft x-ray lasers have the potential to generate a high-energy, highly coherent, short pulse beam. Owing to their high density, plasmas created by interaction of an intense laser with a solid target should store the highest amount of energy among all plasma amplifiers. However, to date output energy from seeded solid amplifiers remains as low as 60 nJ. We demonstrated that careful tailoring of the plasma shape is crucial for extracting energy stored in the plasma. With 1-mm-wide plasma, energy as high as 20 microJ in sub-ps pulses is achievable. With such tailored plasma, gain and pumping efficiency have been increased by nearly a factor of 10 as compared to the narrower plasma amplifiers studied here.